As a process for preparing a 2,6-dihalogenopurine, there have been known, for instance, (A) a process comprising chlorinating xanthine with pyrophosphoryl chloride [J. Am. Chem. Soc. 78, 3508-10 (1956)]; (B) a process comprising chlorinating hypoxanthine or N-oxide of 6-chloropurine with phosphorus oxychloride (Japanese Examined Patent Publication No. Sho 45-11508 and U.S. Pat. No. 3,314,938); (C) a process comprising four steps using a barbituric acid derivative as a starting material [J. Org. Chem. 19, 930 (1954) and J. Am. Chem. Soc. 80, 404-8 (1958)]; (D) a process comprising cyclizing 2,4-dichloro-5,6-diaminopyridine (U.S. Pat. No. 2,844,576); and the like.
However, there are some defects in the above-mentioned process (A) that there is a necessity to prepare pyrophosphoryl chloride as a halogenating agent from phosphorus oxychloride in a complicated method, that a high reaction temperature of 165° C. is required, that a corrosion-resistant reaction vessel is necessitated during the reaction, and that a long period of time of about 19 hours is required for the reaction.
In addition, there are some defects in all of the above-mentioned processes (A) to (D) that their preparation steps are so long, thereby requiring complicated procedures.